The proposed study is an epidemiologic, cross-sectional investigation of 400 homeless shelter users, age 18 years and older, in Washington, DC. The objectives of this study are to: 1) determine the prevalence of drug and alcohol abuse among the homeless, and 2) identify demographic characteristics that relate to drug and alcohol abuse among the homeless. Participants will be selected through a probability based sampling strategy to take part in a 45 minute interview that will take place in the shelter facilities. Data will be gathered on drug and alcohol abuse, homeless state (duration of being undomiciled, previous psychiatric treatment, and current psychiatric status), and demographic characteristics (gender, age, race, marital status, income, occupation, employment status, religious affiliation, and education). Bivariate analysis will be done to examine how homelessness, and demographic characteristics relate to drug and alcohol abuse. The study will take one and a half years. The proposed study will help to further our understanding of the nature and extent of the problem of drug and alcohol abuse among the homeless. In addition, it will contribute to the development of useful prevention strategies and interventions for this high risk population.